Highly effective capture and subsequent catalytic ...

Green Chemistry

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Cite this: Green Chem., 2020, 22,7832

Received 4th September 2020,Accepted 16th October 2020

DOI: 10.1039/d0gc03009k

rsc.li/greenchem

Highly effective capture and subsequent catalytictransformation of low-concentration CO2 bysuperbasic guanidines†

Hui Zhou, * Wei Chen, Ji-Hong Liu, Wen-Zhen Zhang and Xiao-Bing Lu *

Herein, we present a highly efficient and convenient approach for carbon dioxide (CO2) capture and cata-

lytic transformation under mild conditions using N,N’-bis(imidazolyl)guanidines (BIGs, organoguanidine-

based strong superbases) as the organocatalyst, even from simulated flue gas (10% CO2/90% N2, v/v) or

directly from dry air (∼400 ppm CO2). The zwitterionic BIG–CO2 adducts were successfully isolated and

characterized. X-ray single crystal analysis revealed the bent geometry of the binding CO2 in the BIG–CO2

adduct with an O–C–O angle of 129.7° and increased C–O bond distances (1.253 and 1.237 Å) in com-

parison with free CO2. Notably, the resulting BIG–CO2 adducts were found to be capable of catalyzing

the novel cycloaddition of various propiolamidines with simulated flue gas to generate functionalized

(4E,5Z)-4-imino-5-benzylideneoxazolidine-2-ones in good yields and excellent selectivity.

Introduction

Nitrogen base-involving carbon dioxide (CO2) captureand sequestration (CCS), as a kind of climate change miti-gation technology, has attracted so much global attentionwith the aim of reducing anthropogenic CO2 emissions.1

Understanding the interaction pattern of nitrogen bases andCO2 is critical to boost the development of CCS technology(Scheme 1). As is well known, primary and secondaryamines could react with CO2 by rapid nucleophilic attack toform zwitterionic nitrogen base–CO2 adducts (N–CO2

adducts). Due to the inherent instability, N–CO2 adductseasily react with another molecule of an amine via a protontransfer process to form stable ammonium carbamate salts(Scheme 1, I). This method has been widely applied toremove CO2 from highly concentrated and stationary CO2

emission sources, such as power plants and industrialsectors.2 In 2010, the first N–CO2 adduct derived from 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was successfully isolatedand characterized by the Villiers group.3 Note that X-raysingle-crystal diffraction data indicate that the intra-molecular hydrogen bonding leads to an increased stabilityof TBD–CO2 adducts (Scheme 1, II). With the assistance of

Lewis acids, N–CO2 adducts could also be stabilized,thus generating cyclic frustrated Lewis pair (FLP)–CO2

adducts (Scheme 1, III).4 Furthermore, N-heterocyclic imines(NHI) as more electron-rich nitrogen donors were recentlydeveloped to activate the pressured CO2 by the Dielmanngroup and the corresponding stable NHI–CO2 adductswere obtained through a nucleophilic attack process(Scheme 1, IV).5

Scheme 1 Representative methods for CO2 capture and sequestrationby nitrogen base derivatives.

†Electronic supplementary information (ESI) available: Experimental pro-cedures, characterization data, NMR spectra. CCDC No. 1997581 (3b), 1997583(4a), 1997582 (6), 1997584 (8a), 1997585 (8g). For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/d0gc03009k

State Key Laboratory of Fine Chemicals, Dalian University of Technology,

Dalian 116024, China. E-mail: [email protected], [email protected]

7832 | Green Chem., 2020, 22, 7832–7838 This journal is © The Royal Society of Chemistry 2020

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Obviously, the donor strength of Lewis bases is a vital factorfor CO2 activation and sequestration. As a consequence, aseries of Lewis base–CO2 adducts have been synthesized,6

employing strong basic carbon,7 phosphine,8 and oxygenbases.9 N,N′-Bis(imidazolyl)guanidine (BIG) bases, formed bydirect attachment of imidazolyl substituents to guanidinederivatives, have emerged as a new class of nitrogen bases.10

Importantly, the pKa values of the resulting BIG bases weredetermined to be between 26.1 and 29.3 in THF. Althoughcommonly known organoguanidines have widely been appliedfor CO2 capture, activation, and chemical transformation,11 noliterature regarding the use of strong basic BIG systems forCO2 capture and sequestration has been reported. Herein, wereport the synthesis, isolation and structural characterizationof zwitterionic BIG–CO2 adducts via a nucleophilic additionprocess. More importantly, this system is even effective inextracting CO2 from the ambient air. Additionally, we demon-strate that BIG–CO2 adducts also have the ability to catalyzethe novel cycloaddition of propiolamidines with simulatedflue gas to selectively form functionalized (4E,5Z)-4-imino-4-imino-5-benzylideneoxazolidine-2-ones with high activity.

Results and discussionSynthesis and characterization of BIG–CO2 adducts

Firstly, BIG hydrotetrafluoroborates 1a–1d were synthesized aspreviously reported by Ullrich Jahn et al.10a BIG bases were pre-pared by the deprotonation of 1a–1d with KN(SiMe3)2 in THFsolution and further purified by extraction with n-hexane.When the n-hexane solution of BIG bases 2a–2d was placedunder an atmosphere of pure CO2 at room temperature, whiteprecipitates of BIG–CO2 adducts (3a–3d) were rapidly formedand isolated in good to excellent yields (Scheme 2).

Furthermore, BIG–CO2 adducts (3a–3d) were structurallycharacterized by 1H NMR, 13C NMR, IR and MS (ESI).† 13CNMR spectra of 3a–3d show the chemical shifts of the carbonylgroup in the range of 162.1–162.3 ppm, which are quite close

to those of reported NHI–CO2 adducts.10a Meanwhile, a 13Cisotope labeling experiment was conducted, in which the CO2

resonance of 3a (162.1 ppm) obtained from 13CO2 wasenhanced obviously. And also, the CvO stretching frequenciesof 3a–3d were investigated in the range of 1610–1631 cm−1.Gratifyingly, the single crystal of 3a was obtained and deter-mined by X-ray single crystal diffraction, as shown in Fig. 1.The crystal structure data show that the C1–O1 and C1–O2bond distances, 1.253(6) and 1.237(4) Å, respectively, are bothelongated, in comparison with that of free gaseous CO2

(1.160 Å). A bent geometry of the binding CO2 with an O1–C1–O2 angle of 129.78(7)° was observed in the BIG–CO2 adduct,indicating that CO2 is activated through nucleophilic attack byBIG bases. Moreover, the dihedral angles between the plane ofthe guanidine core and the two imidazole planes are 109.85(4)° and 119.54(4)°, respectively.

In addition, the thermal stability of BIG–CO2 adducts 3a–3dwas investigated by means of thermogravimetric analysis(TGA). From the results (ESI, Fig. S1–S4†), the reversible de-carboxylation of 3a–3d began in the range of 129.2–135.8 °C,and the observed weight losses were matched well with thetheoretical content of CO2 in BIG–CO2 adducts.

CO2 capture ability of BIG bases under various concentrationsof CO2

Reversible CO2 capture and release of BIG bases was alsostudied by Density Functional Theory (DFT) calculations (ESI,Fig. S5†). The free energy barriers of BIG bases 2a–2d for CO2

capture are very low (5.5–7.6 kcal mol−1), and the formation ofBIG–CO2 adducts 3a–3c is even more exergonic. Based on thisobservation, BIG bases have the potential to be applied forCO2 activation and capture. Up to now, most of the existingCCS technologies are applied in highly concentrated andstationary CO2 emission sources. Direct CO2 capture fromambient air, by contrast, is becoming more promising to per-manently lower the atmospheric CO2 concentration, thusachieving negative carbon emissions.12 As shown in Fig. 2, theCO2 capture ability of BIG bases with 2a as an example was

Scheme 2 CO2 activation and fixation by BIG bases. Reaction con-ditions: (I) KN(SiMe3)2, THF, 25 °C, 2 h; (II) CO2 (1 atm), n-hexane, 25 °C,2 h.

Fig. 1 POV-Ray illustrations of the molecular structure of BIG–CO2

adduct 3b. Hydrogen atoms have been omitted for clarity. C: black, O:red, and N: dark blue. Selected bond lengths (Å) and angles (°): N1–C1,1.484(6); O1–C1, 1.253(6); O2–C1, 1.237(4); O1–C1–O2, 129.78(7).

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measured under various concentrations of CO2. When thecommercially pure CO2 was introduced with a balloon, a whiteprecipitate was formed immediately to obtain 3a in 84% iso-lated yield within 3 minutes. Moreover, CO2 capture fromsimulated flue gas (10% CO2/90% N2, v/v) could also becarried out at a flow rate of 100 mL min−1, and the corres-ponding 3a was formed in 65% yield within 15 minutes. Whenswitching to dry air containing about 400 ppm of CO2, a yieldof 52% could be obtained with an enhanced reaction time of2 hours.

Protonation of BIG–CO2 adducts in the presence of H2O

Most of the known Lewis base–CO2 adducts are sensitive toprotic solvents, such as water and alcohols.13 Surprisingly,recently reported NHI–CO2 adducts bearing a methyl group atthe exocyclic nitrogen atom showed unprecedented chemicalstability towards hydrolysis, probably due to the hydrophobicnature of the CO2 binding site.5 The stability of BIG–CO2

adducts toward water was also evaluated in THF solution atambient temperature. In the presence of 1.2 equiv. H2O, 2a–2cwas rapidly hydrolyzed via a decarboxylation process to formammonium bicarbonates 4a–c in 90–98% isolated yields.Meanwhile, the X-ray crystal structure of 4a was also deter-mined (ESI, Fig. S7†).

Application of BIG for COS and CS2 capture and activation

Because of the structural similarity of COS and CS2 moleculesto CO2,

14 their capture using BIG base 2a was also investigatedunder the same reaction conditions. As shown in Scheme 3,the COS and CS2 capture processes could smoothly proceed atroom temperature and the corresponding BIG–COS adduct 5and CS2 adduct 6 were isolated in 97% and 98% yields,respectively. The solid-state structure of the BIG–CS2 adduct 6shows that CS2 binds to the nitrogen atom with a N1–C1 bondof 1.427(7) Å and a S–C–S angle of 122.96(4)° (Fig. 3). Note thatthe BIG–CS2 adduct 6 is the first example of a nitrogen-basedzwitterionic adduct. The groups of Vlasse15 and Jessop16 inde-pendently reported the reaction of CS2 with cyclic amidines to

form cyclic carbamic carboxylic trithioanhydride rings, whileacyclic acetamidines were cleaved by CS2 at room temperatureto give an isothiocyanate and a thioacetamide. In addition,Cantat et al. showed that TBD reacted with CS2 and the guani-dinium dithiocarbamate was selectively synthesized via aproton transfer process.17

Application of BIG–CO2 adducts as organocatalysts for CO2

catalytic transformation

As an additional CO2-mitigation strategy to CCS, CO2 captureand utilization (CCU) is attracting global interest.18 Recently,Lewis base–CO2 adducts,

6,19 as a new class of organic catalysts,have exhibited unique reactivity and selectivity for CO2 trans-formation to value-added chemicals, which inspires us tofurther investigate the application of BIG–CO2 adducts in theCCU process. Gratifyingly, BIG–CO2 adduct 3b could efficientlycatalyze the novel cycloaddition of propargylamidine 7a withsimulated flue gas at 80 °C in 24 hours (for detailed optimizedconditions, see ESI, Table S1†), thus generating 4-imino-5-ben-zylideneoxazolidine-2-one 8a in 90% isolated yield. The reac-tions of propargylamidines (7b–7f ) bearing methyl, methoxylor halogen groups (–F, –Cl, and –Br) on the aryl ring gave thecorresponding products 8b–8f in moderate to excellent yields.When the R2 group was transformed to the cyclohexyl group,

Fig. 2 CO2 fixation ability of a BIG base (2a) under various concen-trations of CO2. Reaction conditions: BIG base (2a) (0.1 mmol), hexane(5 mL), 25 °C, CO2 (balloon); simulated flue gas (100 mL min−1); dry air(100 mL min−1).

Scheme 3 COS/CS2 activation and fixation by BIG bases.

Fig. 3 POV-Ray illustrations of the molecular structure of 6. Hydrogenatoms have been omitted for clarity. C: black, S: yellow, and N: darkblue. Selected bond lengths (Å) and angles (°): N1–C1, 1.427(7); S1–C1,1.703(6); S2–C1, 1.680(9); S1–C1–S2, 122.96(4).

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the corresponding substrate 7g was converted to 8g with 90%yield. Meanwhile, the absolute stereostructures of (4E,5Z)-8aand 8g were clearly confirmed by single-crystal X-ray diffractionstudy (Table 1).

To demonstrate the synthetic utility of this transformation,the gram-scale synthesis and further transformations of theproducts were next elucidated, as shown in Scheme 4. Underthe standard conditions, the reaction of 7f (1.2 g) with simu-lated flue gas proceeded smoothly, isolating the corresponding

product 8f in 81% yield (Scheme 4, I). Moreover, the carbon–carbon double bond of product 8a was selectively reduced byH2 in the presence of Pd/C catalyst at room temperature, andthe reduced product 9 was isolated in 95% yield (Scheme 4, II).Oxazolidine-2,4-diones, as a significant class of heterocyclicscaffolds, are frequently found in biologically active and med-icinally useful molecules.20 Note that the hydrolysis of 8f couldtake place smoothly in an aqueous solution of hydrochloricacid, thus affording oxazolidine-2,4-dione 10 in 98% yield(Scheme 4, III).

Conclusions

In summary, we have demonstrated the high efficiency ofsuperbasic guanidines for reversible capture of low-concen-tration CO2 at room temperature and atmospheric pressure,affording the corresponding CO2 adducts in good and excel-lent yields. Moreover, the resulting BIG–CO2 adducts weredemonstrated to be efficient organocatalysts for the cyclizationof CO2 (10% CO2/90% N2, v/v) and propiolamidines to producevarious functionalized (4E,5Z)-4-imino-5-benzylideneoxazoli-dine-2-ones in high yields and excellent selectivity. Thepresent study provides an alternative method for CO2 captureand subsequent catalytic transformation of low-concentrationCO2 under mild conditions. Further explorations regarding theapplications of organocatalytic systems for the synthesis ofvarious heterocyclic chemicals are now in progress.

ExperimentalRepresentative experimental procedure for the synthesis ofBIG–CO2 adducts (3a–3d)

In a nitrogen-filled glove box, BIG salt 1a (545 mg, 1.0 mmol)was added to a suspension of KHMDS (199 mg, 1.0 mmol) inTHF (10 mL) and the mixture was stirred at 25 °C for 2 h. Thenthe solvent was removed in vacuo and the residues wereextracted with n-hexane (10 mL). After filtration to remove theinorganic salt, the filtrate was exposed to 1.0 atm of CO2 atroom temperature for 2 h. The resulting precipitates were col-lected via filtration, washed with n-hexane (3 × 5 mL) and thendried in vacuo to afford BIG–CO2 adduct 3a as a white solid(476 mg, 95% yield). 1H NMR (400 MHz, CDCl3) δ 4.52 (dt, J =14.1, 7.0 Hz, 4H), 4.06 (dt, J = 13.1, 6.5 Hz, 1H), 2.23 (s, 12H),1.47 (d, J = 7.1 Hz, 24H), 1.21 (d, J = 6.5 Hz, 6H); 13C NMR(126 MHz, CDCl3) δ 162.1, 156.2, 147.6, 120.2, 47.9, 44.0, 23.1,21.3, 10.1. IR vCvO: 1650 cm−1. HRMS (ESI): calcd forC27H47N7O2: 458.3966 [M − CO2 + H]+. Found: 458.3962[M − CO2 + H]+.

3b. White solid (464 mg, 90% yield). 1H NMR (500 MHz,CDCl3) δ 4.49 (dt, J = 14.1, 7.0 Hz,4 H), 3.60–3.07 (m, 2H), 2.19(s, 12H), 1.54 (dt, J = 15.0, 7.5 Hz, 2H), 1.44 (d, J = 7.0 Hz,24H), 1.31 (dt, J = 15.0, 7.4 Hz, 2H), 0.86 (dt, J = 14.1, 7.2 Hz,3H). 13C NMR (126 MHz, CDCl3) δ 162.1, 157.4, 147.9, 120.1,47.9, 42.3, 32.5, 21.3, 20.2, 13.9, 10.1. IR vCvO: 1647 cm−1.

Table 1 BIG–CO2 adduct 3b catalyzed cycloaddition of propargylami-dines with CO2 (simulated flue gas)a

aGeneral reaction conditions: Sub. 7 (0.25 mmol), BIG–CO2 adduct 3b(0.025 mmol, 10 mol%), CO2 balloon (10% CO2, 90% N2), DMSO(1.0 mL), 80 °C, 24 h. Isolated yields. b POV-ray depiction of singlecrystal. C: black, O: red, and N: dark blue.

Scheme 4 Gram-scale synthesis and synthetic applications ofproducts.

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HRMS (ESI): calcd for C28H49N7O2: 472.4122 [M − CO2 + H]+.Found: 472.4155 [M − CO2 + H]+.

3c. White solid (434 mg, 82% yield). 1H NMR (400 MHz,CDCl3) δ 4.76–4.24 (m, 4H), 3.23 (s, 2H), 2.25 (s, 12H), 1.48 (d,J = 7.1 Hz, 24H), 0.92 (s, 9H); 13C NMR (101 MHz, CDCl3) δ162.2, 157.3, 147.6, 120.3, 53.6, 47.9, 32.2, 27.4, 21.4, 10.1. IRvCvO: 1654 cm−1. HRMS (ESI): calcd for C29H51N7O2: 486.4279[M − CO2 + H]+. Found: 486.4271 [M − CO2 + H]+.

3d. White solid (542 mg, 82% yield). 1H NMR (400 MHz,CDCl3) δ 4.19 (dt, J = 11.6, 5.6 Hz, 1H), 4.13–3.97 (m, 4H), 2.24(s, 12H), 1.79 (dd, J = 62.7, 9.0 Hz, 28H), 1.44–0.99 (m, 18H);13C NMR (126 MHz, CDCl3) δ 162.3, 155.3, 147.8, 120.6, 56.8,44.1, 31.0, 26.2, 25.1, 23.5, 10.9. IR vCvO: 1639 cm−1. HRMS(ESI): calcd for C39H63N7O2: 618.5218 [M − CO2 + H]+. Found:618.5205 [M − CO2 + H]+.

Representative experimental procedure for the cycloadditionof propiolamidines with simulated flue gas to (4E,5Z)-4-imino-5-benzylideneoxazolidine-2-ones

In a nitrogen-filled glove box, a 10 mL Schlenk flask equippedwith a magnetic stirring bar was charged with propiolamidine7a (57.1 mg, 0.25 mmol), Cat. 3b (12.9 mg, 0.025 mmol,10 mol%) and DMSO (1.0 mL). Then the Schlenk flask wasimmediately transferred from the glovebox, and exchangedwith CO2 (10% CO2/90% N2, v/v) using a balloon. The reactionwas stirred at 80 °C for 24 h. The crude reaction mixture waspurified by column chromatography on silica gel (eluent: pet-roleum ether/ethyl acetate = 5 : 1) to give the desired (4E,5Z)-4-imino-5-benzylideneoxazolidine-2-one 8a (61.2 mg, 90%) as awhite solid. 1H NMR (400 MHz, CDCl3) δ 7.77–7.63 (m, 2H),7.39 (t, J = 7.4 Hz, 2H), 7.32 (t, J = 7.3 Hz, 1H), 6.35 (s, 1H),4.10 (tt, J = 12.3, 3.9 Hz, 1H), 3.84 (t, J = 9.1 Hz, 1H), 2.21 (dq,J = 12.5, 3.3 Hz, 2H), 1.85 (d, J = 9.7 Hz, 6H), 1.68 (t, J = 11.8Hz, 4H), 1.61–1.13 (m, 10H); 13C NMR (101 MHz, CDCl3) δ

152.7, 142.8, 136.1, 132.8, 130.5, 129.0, 128.9, 113.1, 57.0, 52.0,33.9, 28.6, 26.1, 26.0, 25.3, 24.5. IR: 2930, 2855, 1797, 1670,1647, 1450, 1367, 1331, 1232, 1201, 1089. HRMS (ESI): calcdfor C22H28N2O2: 353.2224 [M + H]+. Found: 353.2212 [M + H]+.

8b. White solid (80%). 1H NMR (400 MHz, CDCl3) δ 7.61 (d,J = 7.9 Hz, 2H), 7.20 (d, J = 7.9 Hz, 2H), 6.40 (s, 1H), 4.52 (hept,J = 6.8 Hz, 1H), 4.17 (hept, J = 6.1 Hz, 1H), 2.37 (s, 3H), 1.43 (d,J = 6.9 Hz, 6H), 1.28 (d, J = 6.1 Hz, 6H).13C NMR (101 MHz,CDCl3) δ 152.5, 143.0, 139.4, 135.6, 130.5, 129.9, 129.6, 113.5,49.0, 44.1, 24.1, 21.5, 19.0. IR: 2971, 2935, 1794, 1670, 1647,1407, 1385, 1349, 1330, 1313, 1252, 1178, 1024. HRMS (ESI):calcd for C17H22N2O2: 287.1754 [M + H]+. Found: 287.1752[M + H]+.

8c. White solid (78%). 1H NMR (400 MHz, CDCl3) δ 7.67 (d,J = 8.9 Hz, 2H), 6.91 (d, J = 8.9 Hz, 2H), 6.38 (s, 1H), 4.69–4.36(m, 1H), 4.32–4.08 (m, 1H), 3.84 (s, 3H), 1.42 (d, J = 6.9 Hz,6H), 1.27 (d, J = 6.2 Hz, 6H).13C NMR (126 MHz, CDCl3) δ

160.3, 152.6, 143.1, 134.8, 132.2, 125.5, 114.4, 113.3, 55.5, 49.0,44.1, 24.1, 19.1. IR: 2957, 2927, 1793, 1670, 1646, 1604, 1513,1408, 1385, 1256, 1178, 1026. HRMS (ESI): calcd forC17H22N2O3: 303.1703 [M + H]+. Found: 303.1693 [M + H]+.

8d. White solid (98%). 1H NMR (400 MHz, CDCl3) δ

7.87–7.54 (m, 2H), 7.21–6.87 (m, 2H), 6.38 (s, 1H), 4.52 (hept,J = 7.0 Hz, 1H), 4.31–4.01 (m, 1H), 1.42 (d, J = 7.0 Hz, 6H), 1.27(d, J = 6.2 Hz, 6H). IR: 2972, 1797, 1673, 1650, 1602, 1509,1408, 1386, 1252, 1163, 1023. HRMS (ESI): calcd forC16H19FN2O2: 291.1503 [M + H]+. Found: 291.1493 [M + H]+.

8e. White solid (82%). 1H NMR (400 MHz, CDCl3) δ 7.64 (d,J = 8.6 Hz, 2H), 7.35 (d, J = 8.6 Hz, 2H), 6.36 (s, 1H), 4.51 (m,J = 6.9 Hz, 1H), 4.30–4.00 (m, 1H), 1.42 (d, J = 7.0 Hz, 6H), 1.27(d, J = 6.2 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 152.1, 142.6,136.4, 134.9, 131.6, 131.3, 129.1, 111.9, 49.2, 44.3, 24.1, 19.0.IR: 2972, 1794, 1673, 1648, 1490, 1409, 1385, 1349, 1251, 1180,1080, 1022. HRMS (ESI): calcd for C16H19ClN2O2: 307.1208[M + H]+. Found: 307.1198 [M + H]+.

8f. White solid (90%). 1H NMR (400 MHz, CDCl3) δ 7.50 (d,J = 8.5 Hz, 2H), 7.43 (d, J = 8.5 Hz, 2H), 6.27 (s, 1H), 4.44 (m,J = 6.9 Hz, 1H), 4.08 (m, J = 6.1 Hz, 1H), 1.35 (d, J = 6.9 Hz,6H), 1.20 (d, J = 6.2 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ

152.1, 142.6, 136.5, 132.1, 131.9, 131.7, 123.3, 112.0, 49.2, 44.2,24.1, 19.0. IR: 2970, 2938, 1794, 1671, 1646, 1486, 1408, 1385,1348, 1307, 1250, 1176, 1074, 1021. HRMS (ESI): calcd forC16H19BrN2O2: 351.0703 [M + H]+. Found: 351.0693 [M + H]+.

8g. White solid (90%). 1H NMR (400 MHz, CDCl3) δ

7.77–7.63 (m, 2H), 7.39 (t, J = 7.4 Hz, 2H), 7.32 (t, J = 7.3 Hz,1H), 6.35 (s, 1H), 4.10 (tt, J = 12.3, 3.9 Hz, 1H), 3.84 (t, J = 9.1Hz, 1H), 2.21 (qd, J = 12.5, 3.3 Hz, 2H), 1.85 (d, J = 9.7 Hz, 6H),1.68 (t, J = 11.8 Hz, 4H), 1.61–1.13 (m, 10H). 13C NMR(101 MHz, CDCl3) δ 152.7, 142.8, 136.1, 132.8, 130.5, 129.0,128.9, 113.1, 57.0, 52.0, 33.9, 28.6, 26.1, 26.0, 25.3, 24.5. IR:2930, 2855, 1797, 1670, 1647, 1450, 1367, 1331, 1232, 1201,1089. HRMS (ESI): calcd for C22H28N2O2: 353.2224 [M + H]+.Found: 353.2212 [M + H]+.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 91856108), the FundamentalResearch Funds for the Central Universities (DUT18LK55) andthe Program for Changjiang Scholars and Innovative ResearchTeam in University (IRT-17R14).

Notes and references

1 (a) K. S. Lackner, Science, 2003, 300, 1677–1678;(b) P. G. Jessop, D. J. Heldebrant, X. Li, C. A. Eckert andC. L. Liotta, Nature, 2005, 436, 1102; (c) S. H. Kim,K. H. Kim and S. H. Hong, Angew. Chem., Int. Ed., 2014, 53,771–774; (d) D. M. Reiner, Nat. Energy, 2016, 1, 15011.

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Paper Green Chemistry

7838 | Green Chem., 2020, 22, 7832–7838 This journal is © The Royal Society of Chemistry 2020

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